44 research outputs found
Bose-Einstein condensation of excitons in CuO
We present a parameter-free model which estimates the density of excitons in
CuO, related to experiments that have tried to create an excitonic
Bose-Einstein condensate. Our study demonstrates that the triplet-state
excitons move along adiabats and obey classical statistics, while the
singlet-state excitons are a possible candidate for forming a Bose-Einstein
condensate. Finally we show that the results of this study do not change
qualitatively in a two-dimensional exciton gas, which can be realized in a
quantum well.Comment: 6 pages, RevTex, 1 ps figur
Effectively attractive Bose-Einstein condensates in a rotating toroidal trap
We examine an effectively attractive quasi-one-dimensional Bose-Einstein
condensate of atoms confined in a rotating toroidal trap, as the magnitude of
the coupling constant and the rotational frequency are varied. Using both a
variational mean-field approach, as well as a diagonalization technique, we
identify the phase diagram between a uniform and a localized state and we
describe the system in the two phases.Comment: 4 pages, 4 ps figures, RevTe
Perturbative spectrum of Trapped Weakly Interacting Bosons in Two Dimensions
We study a trapped Bose-Einstein condensate under rotation in the limit of
weak, translational and rotational invariant two-particle interactions. We use
the perturbation-theory approach (the large-N expansion) to calculate the
ground-state energy and the excitation spectrum in the asymptotic limit where
the total number of particles N goes to infinity while keeping the total
angular momentum L finite. Calculating the probabilities of different
configurations of angular momentum in the exact eigenstates gives us a clear
view of the physical content of excitations. We briefly discuss the case of
repulsive contact interaction.Comment: Revtex, 10 pages, 1 table, to appear in Phys. Rev.
Bose-Einstein condensates with attractive interactions on a ring
Considering an effectively attractive quasi-one-dimensional Bose-Einstein
condensate of atoms confined in a toroidal trap, we find that the system
undergoes a phase transition from a uniform to a localized state, as the
magnitude of the coupling constant increases. Both the mean-field
approximation, as well as a diagonalization scheme are used to attack the
problem.Comment: 4 pages, 4 ps figures, RevTex, typographic errors correcte
Vortex lattice of a Bose-Einstein Condensate in a rotating anisotropic trap
We study the vortex lattices in a Bose-Einstein Condensate in a rotating
anisotropic harmonic trap. We first investigate the single particle
wavefunctions obtained by the exact solution of the problem and give simple
expressions for these wavefunctions in the small anisotropy limit. Depending on
the strength of the interactions, a few or a large number of vortices can be
formed. In the limit of many vortices, we calculate the density profile of the
cloud and show that the vortex lattice stays triangular. We also find that the
vortex lattice planes align themselves with the weak axis of the external
potential. For a small number of vortices, we numerically solve the
Gross-Pitaevskii equation and find vortex configurations that are very
different from the vortex configurations in an axisymmetric rotating trap.Comment: 15 pages,4 figure
Landau-Khalatnikov two-fluid hydrodynamics of a trapped Bose gas
Starting from the quantum kinetic equation for the non-condensate atoms and
the generalized Gross-Pitaevskii equation for the condensate, we derive the
two-fluid hydrodynamic equations of a trapped Bose gas at finite temperatures.
We follow the standard Chapman-Enskog procedure, starting from a solution of
the kinetic equation corresponding to the complete local equilibrium between
the condensate and the non-condensate components. Our hydrodynamic equations
are shown to reduce to a form identical to the well-known Landau-Khalatnikov
two-fluid equations, with hydrodynamic damping due to the deviation from local
equilibrium. The deviation from local equilibrium within the thermal cloud
gives rise to dissipation associated with shear viscosity and thermal
conduction. In addition, we show that effects due to the deviation from the
diffusive local equilibrium between the condensate and the non-condensate
(recently considered by Zaremba, Nikuni and Griffin) can be described by four
frequency-dependent second viscosity transport coefficients. We also derive
explicit formulas for all the transport coefficients. These results are used to
introduce two new characteristic relaxation times associated with hydrodynamic
damping. These relaxation times give the rate at which local equilibrium is
reached and hence determine whether one is in the two-fluid hydrodynamic
region.Comment: 26 pages, 3 postscript figures, submitted to PR
Low-lying excitations of a trapped rotating Bose-Einstein condensate
We investigate the low-lying excitations of a weakly-interacting,
harmonically-trapped Bose-Einstein condensed gas under rotation, in the limit
where the angular mometum of the system is much less than the number of the
atoms in the trap. We show that in the asymptotic limit the
excitation energy, measured from the energy of the lowest state, is given by
, where is the number of octupole
excitations and is the unit of the interaction energy.Comment: 3 pages, RevTex, 2 ps figures, submitted to PR
Stress Dependence of Exciton Relaxation Processes in Cu2O
A comprehensive study of the exciton relaxation processes in Cu2O has led to
some surprises. We find that the ortho-para conversion rate becomes slower at
high stress, and that the Auger nonradiative recombination rate increases with
stress, with apparently no Auger recombination at zero stress. These results
have important consequences for the pursuit of Bose-Einstein condensation of
excitons in a harmonic potential.Comment: 10 figures, 1 tabl
Persistent currents in a Bose-Einstein condensate in the presence of disorder
We examine bosonic atoms that are confined in a toroidal,
quasi-one-dimensional trap, subjected to a random potential. The resulting
inhomogeneous atomic density is smoothened for sufficiently strong, repulsive
interatomic interactions. Statistical analysis of our simulations show that the
gas supports persistent currents, which become more fragile due to the
disorder.Comment: 5 pages, RevTex, 3 figures, revised version, to appear in JLT
Temperature-dependent relaxation times in a trapped Bose-condensed gas
Explicit expressions for all the transport coefficients have recently been
found for a trapped Bose condensed gas at finite temperatures. These transport
coefficients are used to define the characteristic relaxation times, which
determine the crossover between the mean-field collisionless and the two-fluid
hydrodynamic regime. These relaxation times are evaluated as a function of the
position in the trap potential. We show that all the relaxation times are
dominated by the collisions between the condensate and the non-condensate
atoms, and are much smaller than the standard classical collision time used in
most of the current literature. The 1998 MIT study of the collective modes at
finite temperature is shown to have been well within the two-fluid hydrodynamic
regime.Comment: 4 pages, 3 figures, to be published in Phys. Rev.